Broad band waveguide directional coupler



Jan. 27, 1959 w. R. HEWLETT 2,871,452

BROAD BAND WAVEGUIDE DIRECTIONAL COUPLER Filed April 23, 1953 2 Sheets-Sheet 1 v INVENTOR. W R He w/e f7 SUPERIMPOSED AR 02 ILDOu .0

SINGLE ARRAY W F" 1 B ATTORNEYS Jan. 27, 1959 w. R. HEWLETT 2,871,452

snow BAND WAVEGUIDE DIRECTIONAL COUPLER Filed April- 23, 1953 2 Sheets-Sheet 2 70 I l I 4: u I] 2OLog (b) Theorel'ical E so 1/ I 3 1 t /2O Log 0 (b) Mea surcd 0 c T T1 2oLo (A)Theore!-ica| 3 20 L0 (A) Measured u -20 a 9 IO u :2 v3

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a a no u i 12 15 I .Q 12 o 1 E 10 b 5 J 3 a a 9 x0 l1 l2 l3 kmc/sec.

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ATTORNEYS BROAD BAND WAVEGUIDE DIRECTIONAL COUPLER William R. Hewlett, Palo Alto, Calif., assignor to Hewlett- Packard Company, Palo Alto, Calif., a corporation of California Application April 23, 1953, Serial No. 350,641

3 Claims. (Cl. 333-40) This invention relates generally to waveguide directional couplers and particularly to rectangular waveguide couplers which have very constant coupling and high directivity over the waveguide band of frequencies.

In the past, directional couplers have most commonly found use in monitoring power levels and for mixing. For such applications the coupling and directivity obtainable from very simple couplers will often sumce. However, for a number of applications, such as the measurement of small reflections, very high performance couplers are required. In typical measurement applications it is desirable to combine a directivity of greater than 30 db with a coupling of from db to db and to maintain both constant coupling and high directivity over as wide a frequency range as possible. In the coupler of the invention it has been possible to satisfy all of these requirements. Over 40 db directivity is obtained in combination with a coupling of 0 db to db, depending on design, and the coupling is flat to within 0.5 db over the entire waveguide band while the directivity stays greater than db over the band. The directivity is generally 10 db greater than has been obtained with previous techniques and the constancy of coupling has been obtained by use of a technique which calls for a minimum of constructional difficulty and close tolerances.

It is thus an object of this invention to provide a directional coupler fora rectangular waveguide which combines high directivity with constancy of coupling over the entire waveguide band of frequencies.

It is a further object of this invention to provide such a coupler having a directivity of greater than 40 db and a coupling which may have a value as low as 0 db with not greater than 0.5 db variations in coupling over the entire waveguide band of frequencies.

It is a further object of this invention'to provide such a coupler which is relatively simple to .construct and States Patent which requires a minimum number of closely 'held tolerances.

Further objects of this invention will appear from the following description in which the preferred embodiment Figure 4 is a perspective view of a single coupling element of the couple'rof the invention.

Figure 5 is a plot of the forward and backward coupling coeiiicients'of'a typical single; element of the type shown Figure 4, as a function of frequency.

; rigure 6 is a "plot of directivity and coupling for u "face'in Figurel l. 1 n l k I I 7 Suppose each coupling element 20 in Figure 2 to be typical complete coupler of the type indicated in Figure 1, as a function of frequency.

In Figure 1 a complete coupler built according to the techm'ques of the invention is shown. A primary metal guide 10 is joined to an auxiliary metal guide 11 along the wide faces of the two rectangular guides. Power is fed in at the opening 12 of the primary guide and a fraction appears at the other opening 13 of the primary guide. The rest of the power is coupled into the auxiliary guide 11 via round holes 14 cut through the common guide face, a part appearing at opening 15 of auxiliary guide 11 and the rest at the other end 16 of auxiliary guide 11. The openings 14 are .of largest diameter for the area midway between the ends of the common guide face, and are of gradually reduced diameter toward the ends of this face. The part of the power transmitted toward the end 16 is absorbed in a resistive load 17, which may typically be a tapered piece of insulating card having a deposited carbon film on one surface. For power supplied at opening 12, much more of the power is coupled in the forward direction in the auxiliary guide 11 to appear at opening 15, than is coupled in the backward direction to be absorbed by load 17. The ratio of the power coupled in the forward direction in the auxiliary guide 11 to the power coupled in the backward direction is called the directivity and is normally express in db. In the coupler of the invention this ratio can be maintained at a valve over 40 db, over the entire Waveguide band of frequencies, i. e. the band accepted in general practice as the maximum range of frequencies over which it is practical to operate a waveguide system. For example, in a l by /2 inch waveguide the generally accepted waveguide band is 8.2 to 12.4 kmc./sec. It is actually possible to operate at slightly lower frequencies and, of course, at any higher frequency, but not without serious changes in the properties of the guide as a transmission system.

The forward coupling ratio or coupling is defined as the ratio in db of the power supplied to the primary guide 10 at opening 12 to the power coupled in the forward direction in the auxiliary guide 11 and appearing at opening 15. In the coupler of the invention it is possible to design for a ratio as low :as 0 db, i. e. substantially all of the power supplied at opening 12 is transmitted to open ning 15. It is also convenient to have couplers with coupling ratios of 3 db, .10 db, and 20 db. With the present invention it has been possible to construct couplers having any of these ratiosand which maintain the coupling constant within 0.5 d-b over the entire waveguide band of frequencies.

To explain the operation of these couplers it is convenient to refer to th'idealized coupled system of- Figure 2. A-firsttransmission line system 13 is coupled to a 'second transmission line system 19 by means of coupling elements 20(1), 20(2), 20(3), 20(N-1), and 20(N). be any number of coupling elements, not shown, up to a total. of N. The coupling elements: are spaced equallyby a distance d alongfboth transmission line systems 18 and 19 which correspond, respectively,,to the primary and auxiliary. guides 10 and 11 in. Figure 1. The coupling elements correspond to the holes 14 inthe common guide identical and alsofassume. that the coupling per element is identical in both' direction's, i. e. for a wave incident,

as V, and'traveling to the right on line 18 each coupling element will induce equal amplitude waves traveling to the leftiand right on line 19. Now; suppose that the distanced is one quarter guide wavelength at'the operatr ing frequency. For this. condition the wave induced on line, 19 by'element 20(1) inthe .forward direction will arrive at element 20(2) in phase with the wave coupled Patented Jan. 27, 1959 The cut indicates that there may by element 20(2) in the forward direction. Waves traveling in the backward direction, however, cancel, because there is a net half-Wavelength path difference between the wave induced at element 20(1) going in the backward direction and the wave induced at 20(1) by the coupling from line 18 to line 19 at element 20(2). Thus the directivity is infinite at this one frequency at which coupling elements are exactly one quarter guide wavelength apart. The directivity would be defined for the system of Figure 2 as 20 log IV /V I for a Wave incident as V coupling per element, one can obtain as complete coupling as desired by simply adding elements. The coupling for the system of Figure 2 would be defined as 20 log {M /V i for an incident wave V Another advantage of a multiplicity of elements becomes apparent when frequency variations are taken into account. it turns out that itis possible to vary the coupling from element to element in such a way that a fairly uniformly high directivity is obtained over a wide band of frequencies instead of a single point of infinite directivity at one frequency with fairly low directivity at frequencies very far from this frequency, as is obtained with equal couplings for all elements. For example, if ten coupling elements are used the coefficients of coupling of the various elements might be chosen to vary from element to element as in Figure 3A which is a plot of coeflicient of coupling versus the number of the element. It is apparent that for the situation considered thus far, i. e. elements which have equal, forward and backward coelficients of coupling and in which the coefficients are not functions of frequency, that the forward coupling, 20 log |V /V would not be a function of frequency. All paths to the output V; are the same length, so it is not important how the wavelength varies with frequency. As will be seen, for a practical system, the coefiicients of coupling of the elements are not necessarily independent of frequency, and in fact great care must be taken to design a system in which the element coupling coefficients are held within desired limits as a function of frequency.

It is sometimes impossible to obtain coeificients of coupling above some specified value. For example, when round holes are used as coupling elements between waveguides, a limitation is met when around 20 db coupling is obtained. To get larger couplings, e. g. of the order of 10 db, ten element arrays require coefficients of coupling from the central holes which correspond to holes overlapping, while if the central holes are held to specified maximum values and the arrays extended in n and length the coupling only increases very slowly, so that impractically long arrays result. These arrays, if used, would theoretically result in excellent directivity, but actually no improvement in directivity is required in going from 20 db to 10 db coupling.

A method of obtaining larger coupling, but with the same directivity as an array of fewer elements, is indicated in Figure 313. Here the coupling coefficients for three ten element arrays have been superimposed to give a 20 element array having coupling coefficients as a function of n as indicated by the heavy upper line in Figure 3B. Over a large part of the distribution the coupling coefiicients are near the maximum. The total coupling is again equal to the sum of the coefiicients, or graphically to the area under the curve of Figure 3B, which is just three times as great as the area under the curve of Figure 3A, and hence corresponds to an increase of coupling by 9.5 db. Combining arrays in this manner theoretically does not alter the directivity at all, since the forward coupling has been increased by exactly the amount that the backward coupling has been increased. In practice a several db improvement in directivity was obtained in going from a 20 db to a 10 db coupler by use of this technique. This method has also been used to build 3 db and db couplers, and it It is also clear that, even with very small tice from its value with a single array, when a superimposed array was employed.

ln addition to the requirements of high directivity and controllable values of coupling which should be capable of being made as low as 3 db with practical elements, it is important for many applications that the coupling be relatively consistant over the frequency range of in terest. In the coupler of the invention this object is accomplished as follows: First, a coupling element is used which couples both electric field and current be tween the two guides, any fairly symmetrical hole being adequate, such as square, round, slightly rectangular, etc. Second, the holes are located oif center at an optimum position to give most constant coupling over the desired frequency range.

Referring to Figure 4, a typical coupling element, a round hole 21, is cut in the common guide wall 22 between guides 23 and 24. The hole 21 has a radius r, and the spacing of the axis of the hole from the nearest side wall of the two guides is X The thickness of the common guide wall is w and this wall is the broad wall having a width 0. Both guides have the same height b.

The forward and backward coupling coefiicients for such a hole are functions of frequency and dimensions. By means of small aperture coupling theory expressions for these quantities have been obtained. The effect of finite wall thickness has been included to a first approximation by means of attenuation factors calculated by regarding the aperture as a cylindrical waveguide beyond cutoff, and calculating the magnetic field coupling using the TE mode and the electric field coupling using thC T1491 mode.

The expressions obtained are as follows:

The three terms in the brackets in the expressions for A and B above arise from the electric field, the transwhere verse component of magnetic field, and the longitudinal component of magnetic field, in that order. For typical values of wall thickness and hole radius, the difference between m and oc is not very important. Assuming these are equal to a and letting X equal (z/4 so that the sin and cos terms in the above will be equal, it is possible to rewrite the expression for A as follows:

'j1|r a{2a Ag} For typical values of a, this quantity is nearly constant over a waveguide band. It thus appears that constant coupling in the forward direction can be obtained from a round hole approximately half way between the center of the guide and the edge. This is convenient for large coupling because two rows of holes may be used parallel to each other, as indicated in Figure 1. v

The' forward and backward coupling coeflicients, A and B, for a round hole between two sections of a 1 by /2 inch waveguide is indicated in Figure 5. The hole diameter was 0.3485 inch and the distance X was 0.200 inch. The theoretical curve for A was slightly flatter than the measured curve, but the measured curve could,

be made more flat by a slight change in X In practice it has proved most practical to start with a theoretical value of X and to 'varyit between several otherwise identical couplers to obtain the optimum by experiment with the complete coupler, rather than to attempt to get the optimum X for each hole individually.

It will be noted that the coefficients of couplings A and B are proportional to the cube of the hole radius r. Thus in the coupler of Figure 1 the hole radii are varied as a function of distance along the guide to provide coupling coefficients which vary in accordance with the Fourier coefficient of the appropriate Tschebyscheff polynomial. 7

It will be noted that the round hole has an inherent directivity which varies from a very low value at the high frequency end of the band to quite substantial values near thelow frequency end. This effect is decidedly beneficial and is reflected in the overall directivity of the coupler, as indicated in Figure 6. Figure 6A shows directivity of two 10 db couplers of the same design as a function of frequency. The coupling curves for the two couplers were essentially the same and the curve is indicated in Figure 6B. It will be noted that the directivity curves show a decided increase at the low frequency end of the band. If the holes had no inherent directivity, an essentially equal ripple directivity curve would be expected over the indicated frequency range. The only reason for the difference between the two directivity curves is small changes in the dimensions of the holes and their positioning. The two couplers for which these curves are shown had both hole diameter and hole positioning held to within $0.001 inch. It will also be noted that the directivity is greater than 46 db over the waveguide band. To make use of this sort of directivity it is necessary to maintain the reflections from the resistive load at one end of the auxiliary guide, 17 in Figure 1, at a very low value. A load with a VSWR of less than 1.01 over the band is used in these couplers.

In the discussion thus far it has been assumed that round holes would be used, but this is not necessary. Theoretically a round hole is very simple, but as a matter of practical design, even when a round hole is used, the final design is based on experimental data, so it would not be particularly difficult to use other than a round hole from a design point of view. It is essential that a hole be used which can have substantially equal magnetic coupling to transverse and longitudinal magnetic fields, in order for the forward coupling to be held constant over a wide range of frequencies. It is also essential that the hole be in the right position to accomplish this purpose. Electric field coupling will normally be present when this is accomplished, although it need not be for this type of coupler to operate. One scheme which substantially eliminates electric coupling is a pair of crossed slots to couple equally to the two magnetic field components, but with very little area to minimize coupling to the electric field. Such techniques, however, are needlessly complex, and in fact would be very difficult to use with comparable accuracy to that obtained in drilling round holes. As indicated by the difference in the two curves of directivity in Figure 6A due to differences less than 0.002 inch in hole position and diameter, the designs used in a coupler with round holes are rapidly approaching a limit practically in which tolerances control available performance. This factor would be made clearly predominant if other forms of holes were used; in particular, if two cuts were require'd p'er coupling element it would not be surprising if 'twice as great accuracy would be required as in a design with one cut. To obtain equal longitudinal and transverse magnetic field coupling with a simple shape hole which may be machined with a single cut, it is necessary that the hole be approximately symmetrical with respect to a line at 45 degrees to the'axis of the guide in a plane parallel to the broad faces of the guides. Although a thin slot at a 45 degree angle would satisfy this symmetry, most forms of hole with this symmetry would include some electric field coupling. In general it is probably true that the simplest type of coupling element is the round hole, and with the higher inherent accuracy obtainable by its use, it is'probable that this form of coupling will be simplest to use in high performance waveguide couplers.

I claim:

l. A rectangular waveguide directional coupler for operation over a waveguide band of frequencies comprising: primary and auxiliary sections of rectangular waveguides having substantially parallel longitudinal axes and being of substantially the same width, said primary and auxiliary sections being joined along one broad wall over a substantial distance to permit coupling between said primary and "auxiliary sections through the common broad wall, a plurality of longitudinally aligned and spaced coupling elements to provide coupling between said primary and said auxiliary sections, each of said coupling elements comprising a hole cut in the common wall between said primary and said auxiliary sections, said hole having an opening which is uninterrupted by the common wall, each of said holes as viewed in plan being substantially symmetrical with respect to a line at 45 degrees with respect to the axes of said primary and auxiliary sections and coplanar with said common wall between said primary and auxiliary sec tions, the spacing between longitudinally aligned coupling elements being not greater than one-half guide wavelength at the highest frequency of said band, the spacing of each coupling element from the center of said common wall being approximately one-quarter of the width of said common wall, thereby to provide substantially equal coupling by each element to the longitudinal and transverse components of magnetic field in said primary section of waveguide over said band of frequencies, and whereby a substantially constant coeflicient of coupling as a function of frequency is obtained for each coupling element over said band.

2. A rectangular wave guide directional coupler for.

operation over a wave guide band of frequencies comprising primary and auxiliary sections of rectangular wave guide having substantially parallel longitudinal axes and being of substantially the same width, said primary and auxiliary sections being joined along one broad Wall over a substantial distance to permit coupling between said primary and auxiliary sections through the common broad wall, a plurality of longitudinally aligned and spaced coupling elements serving to provide coupling between said primary and said auxiliary sections, each of said coupling elements comprising a round hole cut in the common wall between said primary and said auxiliary sections, the spacing between said longitudinally aligned coupling elements being not greater than one-half guide Wavelength at the highest frequency of said band, the spacing of each coupling element from the center of said common wall being approximately one-quarter the width of said common Wall, thereby to provide substantially equal coupling by each element to the longitudinal and the transverse components of magnetic field in said primary section of wave guide over said band of frequencies, and whereby a substantially constant coeflicient of coupling as a function of frequency is obtained for each coupling element over said band.

3. A rectangular Wave guide directional coupler for operation over a Wave guide band of frequencies comprising primary and auxiliary sections of rectangular wave guide having substantially parallel longitudinal axes and being substantially of the same width, said primary and auxiliary sections being joined along one broad wall over a substantial distance to permit coupling between said primary and auxiliary sections through the common broad wall, two coupling arrays, each including a plurality of longitudinally aligned and spaced coupling elements to provide coupling between said primary and said auxiliary sections, each of said coupling elements comprising a hole cut in the common wall between said primary and said auxiliary sections and having an opening which is uninterrupted by the common wall, each of said holes as viewed in plan being substantially symmetrical with respect to a line at 45 with respect to the axis of said primary and auxiliary sections and coplanar with said common wall between said primary and auxiliary sections, the spacing between longitudinally aligned coupling elements of each array beingtnot greater than one-half a guide wavelength at the highest frequency of said band, the spacing of the coupling elements of each array from the center of said common wall being approximately one-quarter of the width of said common wall, with an array on either side of the center of the common wall, thereby to'provide substantially equal coupling by each element to the longitudinal and transverse com- 239 relied on.

ponents of magnetic field in such primary section of wave guide over said band of frequencies, and whereby a suhstantially constant coefficient of coupling as a function of frequencies is obtained for each coupling element over said band.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Barnett and Hunton: A Precision Directional Coupler, H. P. Journal, vol. 3, No. 7-8, copyright April 24, 1952. (Copy in Division 69.)

Rosen et al.: A Consideration of Directivity in Waveguide Directional Couplers, Proc. I. R. B, vol. 37, No. 4, April 1949, pages 393-401.

Le Page et al.: General Network Analysis, McGraW Hill Book Co. Inc., New York, New York, 19S2,'page (Copy in Scientific Library.) 

